Consider using datetime type for 'time' and 'timestamps'.

[danielballan]

We store the times as floating point EPOCH seconds and we present them to the user this way as well. Working with Tatiana recently on data alignment, I found myself wishing for a datetime type, at least in the v2 xarray API. (Changing the type in the documents would be a non-backward-compatible change.)

Related but separate: When we started this project, Mongo had no datetime type. Now it does, and we have proposed to migrate to use it.